Monster thus returns (4)
I believe the answer is:
ogre
'monster' is the definition.
('ogre' can be a synonym of 'monster')
'thus returns' is the wordplay.
'thus' becomes 'ergo' (I've seen this before).
'returns' says the letters should be written in reverse.
'ergo' in reverse letter order is 'OGRE'.
(Other definitions for ogre that I've seen before include "Cruel giant of folklore" , "Fairy tale giant" , "bad-tempered person" , "Bogeyman" , "Terrifying monster" .)